1. Field of the Invention
The present invention relates to a color imaging element and an imaging device and, in particular, relates to a color imaging element capable of reducing occurrence of color moire and achieving high resolution, and an imaging device using the color imaging element.
2. Description of the Related Art
In a single-plate color imaging element, a single color filter is provided on each pixel, and hence each pixel has color information about only a single color. For this reason, an output image of a single-plate color imaging element is a RAW image (mosaic image), and thus multichannel images are obtained by a process (demosaicing process) of interpolating a pixel of a missing color from surrounding pixels. What matters in this case is reproduction characteristics of a high frequency image signal. The color imaging element is likely to cause aliasing in a captured image in comparison with a monochrome imaging element, and thus it is important to achieve a high resolution by expanding a reproduction band while reducing occurrence of color moire (false color).
In a primary color Bayer array as a color array of color filters most widely used in the single-plate color imaging elements, green (G) pixels are placed in a checkered pattern, and red (R) and blue (B) pixels are disposed line-sequentially. Thus, there is a problem with reproduction precision when G signals generate high frequency signals in diagonal directions, and when R and B signals generate high frequency signals in horizontal and vertical directions.
Assume a case where a monochrome vertical stripe pattern (high frequency image) as indicated by the (A) portion in FIG. 13 is incident on a color imaging element including color filters in a Bayer array indicated by the (B) portion in FIG. 13. When this pattern is disposed in a Bayer color array and compared for each color, R forms a light and flat color image, B forms a dark and flat color image, and G forms a light and dark mosaic color image as indicated by the (C) to (E) portions in FIG. 13. There is no density difference (level difference) between RGB with respect to the original monochrome image, but the image is colored depending on a color array and an input frequency.
Similarly, assume a case where a diagonally monochrome high frequency image as indicated by the (A) portion in FIG. 14 is incident on an imaging element including color filters of a Bayer array indicated by the (B) portion in FIG. 14. When this pattern is disposed in a Bayer color array and compared for each color, R and B form light and flat color images, and G forms a dark and flat color image as indicated by the (C) to (E) portions in FIG. 14. Assuming that a value of black is 0 and a value of white is 255, the diagonally monochrome high frequency image turns green since only G is 255. Thus, the Bayer array cannot correctly reproduce a diagonal high frequency image.
Generally, in an imaging device using single-plate color imaging elements, optical low pass filters made of a birefringent material such as crystal are placed in front of the color imaging elements to optically reduce a high frequency wave, thereby avoiding this problem. This method can reduce coloring due to aliasing of a high frequency signal, but has a problem that the resolution lowers due to a negative effect of this method.
In order to solve such a problem, a color imaging element has been proposed, wherein a color filter array of the color imaging element is a three-color random array satisfying array limitation conditions in which arbitrary pixels of interest are adjacent to three colors including colors of the pixels of interest on any of four sides of the pixels of interest (Japanese Patent Application Laid-Open No. 2000-308080: PTL 1).
In addition, an image sensor (color imaging element) with a color filter array has been proposed, wherein the image sensor includes a plurality of filters having different spectral sensitivities and in which first filters and second filters are alternately disposed in a first period in one of diagonal directions of a pixel grid of the image sensor, while the first filters and second filters are alternately disposed in a second period in the other one of the diagonal directions (Japanese Patent Application Laid-Open No. 2005-136766: PTL 2).
Further, a color array has been proposed, wherein the color array in a color solid state imaging element (color imaging element) of three primary colors of RGB equalizes appearance probabilities of each RGB by disposing sets of three pixels of horizontally-disposed R, G and B in a zig-zag pattern in the vertical direction, and allows arbitrary straight lines (horizontal, vertical and diagonal lines) on an imaging surface to pass through all colors (Japanese Patent Application Laid-Open No. 11-285012: PTL 3).
Furthermore, a color imaging element has been proposed, wherein R and B of the three primary colors of RGB are disposed every third pixel in the horizontal direction and in the vertical direction, and G is disposed between the R and B (Japanese Patent Application Laid-Open No. 8-023543: PTL 4).